1. Field of the Invention
The present invention relates to an electrically operated power steering apparatus for producing assistive steering forces to assist the driver of a motor vehicle in steering the motor vehicle.
2. Description of the Prior Art
One known electrically operated power steering apparatus which employs an electric motor to assist in steering a motor vehicle is disclosed in Japanese laid-open patent publication No. 3-7661, for example. The disclosed electrically operated power steering apparatus includes a power circuit spaced from the electric motor and connected to the field coil of the stator of the electric motor by long motor wires.
Generally, electrically operated power steering apparatus on motor vehicles detect a manual steering force from a steering wheel with a torque sensor, energize a brushless motor with a detected signal from the torque sensor, and converts the rotation of the brushless motor into linear movement of a steering shaft for producing an assistive force to be added to the manual steering force to steer the motor vehicles. For example, a conventional electrically operated power steering apparatus shown in FIG. 14(b) of the accompanying drawings has a torque sensor that supplies a detected signal to a central processing unit (CPU), which applies a control signal to a motor control circuit that controls a power circuit to energize a brushless motor. The power circuit comprises a plurality of power devices which are typically metal-oxide-semiconductor field-effect transistors (MOS-FETs).
Heretofore, the power circuit is included in a control unit which is positioned separately from the brushless motor. Therefore, power lines or electric wires connecting the power circuit to the stator of the brushless motor are exposed therebetween and relatively long. The long electric wires cause a large resistance-induced energy loss, and tend to produce noise impairing the function of the power devices of the power circuit because of high-speed switching of large currents flowing through the electric wires.
One solution would be to minimize the length the electric wires connecting the power circuit to the brushless motor. If the electric wires were shortened, then the power devices would have to be positioned closely to the stator of the brushless motor. Since the power devices are susceptible to heat, it would be necessary to rely on some measures to radiate the heat that is generated by the stator as well as the heat generated by the power devices themselves.